1. Field of the Invention
The present invention relates to a method of restoring a dual transmission line which connects respective stations to make a local-area network.
2. Description of the Prior Art
FIG. 6 shows a conventional ring local area network having three stations S1, S2, and S3 connected to each other with a primary transmission line 1 and a secondary transmission line 2 at respective primary and secondary input and output terminals PI, PO, SI, and SO. Each station has a computer and other equipment, and these machines are able to communicate each other through the stations. The data are circulated along the ring transmission line in the direction of an arrow. Normally, only the primary line 1 is used, with the secondary line 2 standing by for use when the primary line has a fault.
FIG. 7 shows the fields of a frame which flows in a ring local-area network such as shown in FIG. 6. This frame has a start delimiter SD indicating the beginning of the frame, a destination station address DA, a source station address SA, a command C indicating the frame type, a user data I to be added if necessary, a frame check sequence FCS for detecting a frame error, an end delimiter ED indicating the end of the frame, and a frame status FS indicating the response.
For example, the station 2 transmits a frame of respective fields SD through ED at the primary output PO on the primary transmission line 1. The frame is circulated along the primary line 1 and returned to the same station 2 at the primary input PI, where it is discarded. The other station S1 or S3 constantly monitors the frame on the primary line 1 while repeating it and captures the contents of the frame addressed to the station and transmits the result of the capture in the FS field. The transmitting station S2 checks the FS field to see whether the transmission is completed. The transmitting station must obtain the right to transmit according to the procedure determined by the system, but this procedure has nothing to do with the present invention and its description will be omitted.
A conventional method of restoring a transmission line will be described with reference to FIG. 8 in which the primary line 1 is broken between the stations S1 and S2. When the primary line 1 is broken between the stations S1 and S2 causing a fault and the station S2 detects the carrier breakdown at the primary input PI on the primary line 1, it connects the primary input PI to the secondary output SO and transmits without interruption through the secondary line 2 to the adjacent station S1 a special symbol instructing that the primary output PO be connected to the secondary input SI. The station S1 receives the special symbol at the secondary input SI on the secondary line 2 and connects its primary output PO to its secondary input SI as instructed. In this way, the transmission line of the ring local-area network is restored to continue communications.
The condition that the secondary output is connected to the primary input is hereinafter called a "WRAPA" state, that the primary output is connected to the secondary input, a "WRAPB" state, both WRAPA and WRAPB states, generally a "wrap" or "loopback" state, and the other or non-loopback condition, a "THRU" state.
In the above example, the transmission line is completely broken to cause a carrier breakdown, but there is another kind of fault that only the contents of information are broken without causing a carrier breakdown (hereinafter called "information transmission fault"). At present, there is no method of restoring a transmission line from such a fault, but only the fault location is able to evaluate.
A conventional method of evaluating a fault location will be decribed with reference to FIG. 9. Each station S1, S2, or S3 has a nomal mode 11 and a beacon mode 12. In the normal mode, when there is no information transmission trouble on the transmission line, each station performs a normal transmission reception operation while monitoring a control symbol which is generated on the transmission line in a predetermined period of time. In the beacon mode, when a fault occurs on the transmission line, a station detects the fault by monitoring the control symbol and transmits to the adjacent downstream station a beacon frame indicating the transmission fault. When the station in the beacon mode receives the beacon frame, it returns to the normal mode.
A method of restoring a transmission line from an information transmission fault will be described with reference to FIG. 10. When a fault occurs on the primary line 1 between the stations S1 and S2, the respective stations in a normal mode are unable to detect a control symbol within a predetermined period of time and go to a beacon mode about the same time. The respective stations S1, S2, and S3 in the beacon mode transmits a beacon frame to the adjacent downstream station as indicated by a broken line 100. The station S3 receives the beacon frame from the station S2 to return to the normal mode, and the station S1 receives the beacon frame from the station S3 to return to the normal mode, but the station S2 is unable to receive the beacon frame from the station S1 because of the information transmission fault on the transmission line 1 and remains in the beacon mode. Hence, the system is brokendown, but it is only possible to judge that the fault location lies between the stations S2 in the beacon mode and its upstream station S1. In the above beacon frame, the DA and FS fields are insignificant and the I field does not exist.
As has been described above, the conventional method of restoring a transmission line is able to correct a carrier breakdown but unable to correct an information transmission fault without carrier breakdown except for providing an approximate fault location.